This invention relates to a method for reducing the carbon content of siliceous combustion ash and related products and a related apparatus. More specifically, the invention relates to a method for removing carbon from coal combustion fly ash or similar materials in a manner that improves the pozzolanic properties of the resultant ash and products related thereto and a related apparatus.
It is well known that fine siliceous ash, especially fly ash resulting from coal combustion, has found widespread commercial use as a substitute for Portland cement in concrete because of the pozzolanic properties of such ash. The use of siliceous ash in this application requires that the carbon content of the ash be generally less than 6% by weight and preferably 4% or less by weight in order to obtain the required pozzolanic properties. This requirement results from the fact that the properties of concrete incorporating high-carbon ash are inferior to those of concrete incorporating low-carbon ash and the amount of water and quantity of air entraining agents used in the mix must be increased significantly as the carbon content of the ash increases above the 6% level.
It is also well known that the implementation of more stringent air quality regulations pertaining to coal-burning power plants and other industrial combustion operations has resulted in the installation and use of combustion gas purification technologies including burners that generate lower levels of nitrogen oxides in the combustion gases discharged to the atmosphere (low NOx burners). The use of such burners significantly increases the carbon content and alters the characteristics and pozzolanic properties of the resulting ash. The application of these new burners has significantly decreased the quantity of available ash that is suitable for use as a constituent in concrete and shaped building products.
The amount, size, and shape of the carbon particles, the quantity of carbon particles trapped within or tightly adhered to the siliceous particles in the ash, the quantity of siliceous particles trapped within or tightly adhered to carbon particles within the ash, and the pozzolanic properties of the siliceous particles within an ash vary widely from ash-to-ash depending upon the composition of the coal and the combustion parameters and pollution technology employed in its combustion. Optimum carbon removal from a given ash requires removal of a significant portion of the free carbon particles from the ash, liberation and removal of a portion of the entrapped or tightly adhered carbon particles, liberation and retention of the entrapped or tightly adhered siliceous particles, and minimizing the quantity of siliceous material removed along with the carbon.
A number of methods and apparatus have been developed and commercialized for use in removing carbon particles from fly ash, including methods based upon combustion of the carbon and those based upon removal of the carbon particles by mechanical, electrostatic, gravitational, and flotational means and combinations thereof.
Combustion means for removing carbon from fly ash by thermally oxidizing a significant portion of the free carbon in the ash via heating in an atmosphere of air and/or oxygen are described in U.S. Pat. No. 5,555,821, and U.S. Pat. No. 5,390,611. Similarly, U.S. Pat. No. 5,160,539 and U.S. Pat. No. 5,399,194, disclose reducing the carbon content in fly ash by introducing the fly ash/carbon mixture into a dry bubbling fluid bed and supplying air thereto at temperatures sufficient to combust the carbon. All of these thermal methods, although effective in reducing the amount of carbon present in the fly ash, are energy intensive, involve costly material handling procedures, and reduce the possibility of recovering any carbon values from the ash.
Mechanical means of removing carbon from siliceous ash based upon the relative particle size of the carbon particles and the siliceous particles in the ash is disclosed in U.S. Pat. No. 5,797,496 which discloses the separation of the coarser carbon particles from the finer siliceous particles by passing the fly ash mixture through screens of varying mesh size.
A gravitational means for removing carbon from a siliceous ash/carbon mixture disclosed by Groppo, J. G., Brooks, S. M., and Krieser ("Fly Ash Beneficiation By Air Classification ("Fly Ash Beneficiation By Air Classification", SME Annual Meeting, March, 1995) involves subjecting the mixture to an air stream in which the larger, less dense carbon particles are separated from the smaller, more dense siliceous particles. Other gravitational separation means are described in U.S. Pat. No. 3,769,054 which discloses subjecting a fly ash/carbon mixture to air classification in order to achieve a low-carbon fraction and a high-carbon fraction and screening the high-carbon fraction, and U.S. Pat. No. 5,299,692 which discloses a separation method based upon vibrating a fly ash/carbon mixture on an inclined surface to cause the light high-carbon fraction to rise to the top and the heavy-low carbon siliceous fraction to gravitate to the bottom of the mixture while subjecting the top of the mixture to a directional flow of air that removes the high-carbon fraction from the mixture and causes it to be collected in collection chambers. A two-stage process for removing carbon from the coarse fly ash fraction resulting from an initial air classification of an as-combusted ash, reported in the "Transactions, Materials Research Society of Japan, Volume 18A" ("Experiences in Separating Residual Carbon in Coal Fly Ash by Grinding and Air Classification"), involves subjecting the high-carbon coarse fraction from the initial air classification step to a second step in which mechanical grinding is combined with air classification in a manner that results in selectively grinding the free carbon particles in the coarse fraction without reducing the particle size or affecting the characteristics of the ash particles in coarse fraction.
Electrostatic means for removing carbon from a fly ash/carbon mixture are disclosed in U.S. Pat. No. 4,357,234, U.S. Pat. No. 4,514,289, and U.S. Pat. No. 4,517,078. Generally, in each of these processes, separation is achieved by subjecting the fly ash/carbon mixture to a directional electrostatic force which causes the lighter, more highly charged carbon particles to move in a direction different from that of the heavier, less charged siliceous particles thereby dividing the material into a high-carbon fraction and a low-carbon fraction. Another electrostatic method for removing carbon from fly ash, disclosed in U.S. Pat. No. 4,556,481, involves maintaining the fly ash/carbon mixture in a fluidized state while subjecting the fluidized material to an electrostatic field that imparts a centrifugal force to the more highly charged carbon particles and causes the carbon-rich fraction to move in a direction different from that of the low-carbon siliceous fraction. U.S. Pat. No. 4,115,256 discloses electrostatic separation of carbon and siliceous ash particles including upgrading the carbon fraction by passing said fraction through a cross-flow of ionized air.
A means of removing carbon from fly ash by flotational means, disclosed in U.S. Pat. No. 4,121,945, involves initially screening a fly ash/carbon mixture to remove coarse particles, removing a significant portion of the carbon from the mixture via froth flotation to create a low-carbon siliceous fraction, and grinding the low carbon siliceous fraction to further improve its pozzolanic properties. U.S. Pat. No. 4,426,282, U.S. Pat. No. 5,047,145, and U.S. Pat. No. 5,227,047 describe multi-step methods in which the carbon content of fly ash/carbon mixtures is reduced using wet flotation means which do not involve initial coarse screening.
Although all of the aforementioned may be used to remove carbon particles from siliceous fly ash, the efficiency and degree of removal achieved and the pozzolanic properties of the resulting low-carbon siliceous fraction are highly dependent upon the physical characteristics of the original ash. Further, these methods are not capable of efficiently recovering carbon entrapped in or tightly adhered to the siliceous ash particles or freeing and retaining siliceous material combined with or tightly adhered to carbon particles.
There remains, therefore, a need for an energy efficient, effective, economical method and apparatus for removing and recovering free and combined carbon from a fine ash/carbon mixture that increases the yield of both the low-carbon siliceous fraction and recovered carbon and improves the pozzolanic properties of the siliceous fraction in order to enhance its application as a constituent in concrete, shaped building products, and other materials of construction. The availability of such a method and apparatus would greatly increase the value and level of consumption of processed ash utilized as an additive in concrete and significantly decrease the volume of ash that must be disposed in landfills.